This invention relates to an electrically heatable sheet prepared by paper making technique using carbon fibers, and a method for preparing the same.
In the prior art, there are a variety of proposals with respect to an electrically heatable sheet containing carbon fibers and adapted to be utilized in, for example, floor heaters, horticultural equipments, bedding, health appliances or cattle sheds. For example, there is disclosed in Japanese Unexamined Patent Publication No. 18702/1975 a paper-like composition containing acrylonitrile type synthetic resin pulp and carbon fibers. This paper-like composition lacks in practical utilities since the carbon fibers cannot be mixed and entangled uniformly such that the electrical resistance of the electrically heatable sheet formed of the paper-like composition is not uniform and hence the sheet cannot be increased in size.
There is also disclosed in Japanese Unexamined Patent Publication No. 281293/1987 an electrically heatable sheet containing short cut carbon fibers and natural pulp, wherein the fiber surface is coated at least partially by solid contents of a viscous or glueing agent. This prior-art sheet contains the solid contents of the glueing agent as the essential component for improving dispersibility of the carbon fibers and entanglements between the carbon fibers and natural fibers. However, there is a limit to the amount of the solid contents of the glueing agent in the composition such that the carbon fibers cannot be dispersed sufficiently and the electrical resistance cannot be rendered uniform throughout the electrically heatable sheet. Also the carbon fibers are not entangled sufficiently in the sheet so that it is difficult to raise the sheet temperature to a temperature of higher than 50.degree. C. Moreover, as the carbon fibers employed in the electrically heatable sheet are cut previously to a predetermined size, it takes much time until the fibers are dispersed in the sheet. In addition, the fiber size may become discrete due to fiber breakage at the time of dispersion, resulting in insufficient entanglement and non-uniform electrical resistance and temperature. For improving the mechanical strength of the sheet, it is proposed to coat and seal both sides of the electrically heatable sheet with a plastic film or sheet. Although the tensile strength of the sheet can be increased in this manner by the sole proposal of the prior art, the bending strength of the sheet may be lowered, while the usage of the sheet is also restricted. That is, only the sheet surface is coated, while the structure of the sheet itself is not improved, and disadvantages are presented since the sheet itself may be destitute in pliability by such coating treatment of the sheet surface.
On the other hand, in the electrically heatable sheet employing plant or natural pulp, disadvantages are also presented in connection with the thermal resistance of the sheet itself, such that the sheet temperature cannot be raised to a temperature of higher than 60.degree. C. and particularly to a temperature of higher than 100.degree. C., while temperature characteristics are extremely unstable at higher temperatures.
In the meanwhile, although silver paste electrodes have been used as the electrodes for the electrically heatable sheet, the heating temperature may be varied depending on the mounting positions of these electrodes, so that electrically heatable sheet as a whole cannot be maintained at a constant temperature. There is also an additional disadvantage that, when the electrical resistance of the sheet disposed between the electrodes is lower than the electrical resistance of the electrode itself, the electrode may be heated to evolve heat.